Rotary drilling is the most common drilling technology practiced in the oil patch today. Although drilling operations have matured, the increase of economic interests by maximizing the oil recovery capacity have led towards more complex drilling operations, such as deep water drilling, multi-well drilling and horizontal drilling. To accomplish the drilling operations as rapidly and as economically as possible, a great deal of planning and forethought must be put into the drilling program. Among many other parameters, the design of well-adapted drilling fluids is one of the most critical factors for the success of the drilling operations. The drilling fluids are primarily dependent upon the geological formation being drilled and the problems associated with such type of formation.
During the drilling operations, the drilling fluid is injected into the well through the drill pipe and re-circulated to the surface in the annular area formed by the wellbore wall and the drill string. Once back at the surface, the drilling fluid is physically and chemically treated and conditioned before it is pumped back into the well.
The drilling apparatus is comprised of a column of drill pipes to the bottom of which is attached a multi-pronged drill bit. To drill the ground, the column of drill pipes is rotated to cut the earth. As drilling continues, cuttings are accumulated. To facilitate drilling, these cuttings have to be continuously removed from the vicinity of the drill bits at the bottom of the hole. The drilling fluid is pumped down-hole through the drill pipe and pumped through nozzles at the drill bit. The generated cuttings are removed from the down-hole to the surface by the drilling fluid through the annular space between the formation and the drill pipe. Depending on whether the continuous phase is water, oil, pseudo-oil, or gas, the drilling fluid is called water-based, oil-based, pseudo-oil based or foam-based. Water-based drilling fluids are preferred over oil-based or pseudo oil-based for economic and environmental reasons.
According to American Petroleum Institute (API), drilling fluid is defined as a circulating fluid used in rotary drilling to perform any or all of the various functions required in drilling operations (J. L. Lummus and J. J. Azar, in “Drilling Fluids Optimization: A Practical Field Approach”; Penn Well Publishing Company: Tulsa Okla., pp. 19-27, 1986). The main functions of the drilling fluid are to: (i) cool, clean and lubricate the drill bit, (ii) suspend the drill cuttings from the drilling operations, (iii) carry them to the surface and remove them from the bottom of the borehole, (iv) prevent loss of excessive amounts of fluids from flowing from the hole into surrounding formations by forming on the wall of hole a thin and impervious filter cake, (v) serve as a fluid column to exert sufficient hydrostatic pressure to counterbalance the formation pressure (water, gas, oil or other earth fluid) and (vi) prevent caving or other intrusions into the drill hole.
To perform these functions, an efficient drilling fluid must exhibit numerous characteristics, such as desired rheological properties (plastic viscosity, yield value and low-end rheology, gel strengths), fluid loss prevention, stability under various temperature and pressure operating conditions, stability against contaminating fluids, such as salt water, calcium sulfate, cement and potassium contaminated fluids, etc. A wide variety of additives are added to drilling fluid formulations to achieve the above performance properties. These include soluble or insoluble inorganic and organic species, and water-soluble or-swellable clays and polymers.
Drilling fluids, often referred to as muds or drilling muds in the oil industry (both terms will be used interchangeably in the present document), can be classified on the basis of their principal constituent (continuous phase). The continuous phase may be water, oil, pseudo-oil and gas. The resulting drilling fluids are called water-based muds (WBMs), oil-based muds (OBMs), synthetic or pseudo-oil based muds (SBMs or POBMs, respectively) and foam muds, respectively. Depending upon the type of the drilling fluid, the continuous phase may additionally contain dissolved organic and inorganic additives and suspended, finely divided solids of various types. Each of these muds has advantageous and disadvantageous features.
Oil-Based Muds
They have oil as the continuous phase. The oil most often selected is diesel oil, mineral oil and low toxicity mineral oil. Because some water will always be present, the oil-based mud must contain water-emulsifying agents. If water is purposely added (for economical reasons), the oil-based mud is called an “invert emulsion mud”. Various thickening and suspending agents as well as barite are added. The emulsified water may contain alkalies and salts.
Due to their continuous phase, OBMs are known to provide unequaled performance attributes with respect to the rate of penetration, shale inhibition, wellbore stability, high lubricity, high thermal stability and high salt tolerance. However, they are subjected to strict environmental regulation regarding their discharge and recycling.
Pseudo Oil-Based Muds (Synthetic Oil-Based Muds)
POBMs (or SBMs) are made on the same principle as OBMs. They have been developed to maintain the performance characteristics of OBMs while reducing their environmental impact. The objective behind the design of these drilling fluids is to exchange the diesel oil or mineral oil base with an organic fluid which exhibits a lower environmental impact. The organic fluids used are esters, poly-olefins, acetal, ether and linear alkyl benzene. As with OBMs, POBMs may contain various ingredients, such as thickening and suspending agents, emulsifying agents as well as weighting agents.
POBMs were developed to technically maintain the performance characteristics of OBMs while reducing their environmental impact. They are, however, not as stable as OBMs depending upon the continuous phase. From environmental perspective, the current legislation is becoming as strict for POBMs as for OBMs.
Gas-Based Muds
Although natural gas (methane), exhaust or combustion gases can be used, air is the most common gas drilling fluid. Air is used to produce the so-called “Foam Muds” in which air bubbles are surrounded by a film of water containing a foam-stabilizing substance or film-strengthening materials, such as organic polymers or bentonite. This type of muds is not re-circulated and is particularly used for reduced-pressure drilling to improve the hole stability in caving formations. However, this type of muds has some use limitation with respect to drilling water-producing or wet formations as well as a limited salt tolerance.
Conventional Water-Based Muds
They have water as the continuous phase. Water may contain several dissolved substances. These include alkalies, salts and surfactants, organic polymers in colloidal state, droplets of emulsified oil and various insoluble substances, such as barite, clay and cuttings in suspension. The mud composition selected for use often depends on the dissolved substances in the most economically available makeup water or on the soluble or dispersive materials in the formations to be drilled. Several mud “types” or “systems” are recognized and described in the literature such as, but not limited to: spud muds, dispersed/deflocculated muds, lime muds, gypsum muds, salt water muds, non-dispersed polymer muds, inhibitive potassium muds, cationic muds and mixed metal hydroxide (MMH) muds.
Despite their environmental acceptance, conventional WBMs exhibit major deficiencies relative to OBMs/POBMs with regard to their relatively poor shale inhibition, lubricity and thermal stability characteristics. To overcome those deficiencies, specific additives may, however, be added into the WBM compositions to deliver properties approaching, even though not as good as OBMs/POBMs performance while minimizing the environmental impact.
Consequently, to meet the new environmental regulations while extending the technical performance of water based drilling fluids, a new generation of water based fluids, also called “inhibitive drilling fluids” was developed to compete against OBMs. Also, to minimize the formation damage, new types of non-damaging drilling fluids, called “drill-in-fluid”, have been developed to drill the pay-zone formations
Inhibitive Water-Based Muds
Minimizing the environmental impact of the drilling process is a highly important part of drilling operations to comply with environmental regulations that have become stricter throughout the world. In fact, this is a mandatory requirement for the North Sea sector. The drilling fluids industry has made significant progress in developing new drilling fluids and ancillary additives that fulfill the increasing technical demands for drilling oil wells. These additives have very little or no adverse effects on the environment or on drilling economics.
New drilling fluid technologies have been developed to allow the continuation of oil based performance with regard to formation damage, lubricity and wellbore stability aspects and thus penetration rates. These aspects were greatly improved by incorporating polyols or silicates as shale inhibitors in the fluid systems.
Polyols based fluids contain a glycol or glycerol as a shale inhibitor. These polyols are commonly used in conjunction with conventional anionic and cationic fluids to provide additional inhibition of swelling and dispersing of shales. They also provide some lubrication properties.
Sodium silicates or potassium silicates are known to provide levels of shale inhibition comparable to that of OBMs. This type of fluids is characterized by a high pH (>12) for optimum stability of the mud system. The inhibition properties of such fluids are achieved by the precipitation or gelation of silicates on contact with divalent ions and lower pH in the formulation, providing an effective water barrier that prevents hydration and dispersion of the shales.
Drill-In-Fluids
After drilling a well to the total depth, it is a normal practice to replace the drilling mud with a completion fluid. This fluid is a “clean”, solids-free, (or acid soluble), non-damaging formulation, intended to minimize reductions in permeability of the producing zone. Prior to producing from the formation, it is usually necessary to clean up what is left by the original mud and the completion fluid, by breaking and degrading the filter cake with an oxidizer, enzyme or an acid solution.
Nowadays, many wells exploit the pay-zone formations horizontally and for long distances. It is no longer practical in these wells to drill the pay-zone with conventional solids-laden muds as the extended clean-up process afterwards is much more difficult. Consequently, the modern generation of drill-in-fluids were developed.
Drill-in-fluids are completion fluids, but they also act as drilling muds. As the pay-zone is penetrated horizontally, these fluids must provide the multifunctional requirements of drilling fluids in addition to the “non-damaging” attributes of completion fluids. In practice, therefore, the normal drilling mud is replaced with a drill-in-fluid just before the pay-zone is penetrated, and used until the end of operations.
Fluid loss prevention is a key performance attribute of drilling fluids. For water-based drilling fluids, significant loss of water or fluid from the drilling fluid into the formation can cause irreversible change in the drilling fluid properties, such as density and rheology occasioning instability of the borehole. Fluid loss control is measured in the laboratory according to the standard procedure (API-RP-13B) for testing drilling fluids.
A wide variety of water-soluble or water-swellable polymers, such as cellulosics, starches, guar gums, xanthan gums, and synthetic polymers and copolymers of acrylamide, acrylic acid, acrylonitrile, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) are used in water-based drilling fluids. The most commonly used polymers used to build viscosity are the cellulosics, guar gums, xanthan gums, and polyacrylates. Among cellulosics, carboxymethylcellulose (CMC), polyanionic cellulose (PAC) and carboxymethylhydroxyethylcellulose (CM-HEC) are used. Among them, CMC and PAC are the most preferred ones based on their cost and performances to control viscosity (often combined with xanthan gum) and fluid retention (often combined with starches) during well drilling. They are generally available in two main grades, designated as high viscosity (“Hivis”) and low viscosity grades (“Lovis”). Hivis PACs are primarily used for viscosity control and secondarily for fluid loss control while Lovis PACs are primarily used as fluid loss reducer; Lovis PACs are not supposed to contribute to the viscosity of the drilling fluid system. In most cases, both types are used together in a drilling fluid composition. During drilling operations, optimum drilling fluid attributes are further achieved by combining different types of additives such as CMC, PAC, xanthan gum (primary rheology modifier), starches (improved filtration control) and other synthetic polymers that may be required for dispersing or shale inhibition properties.
U.S. Pat. No. 5,028,342 describes the use of a mixture of CMC made from raw linters and polycarboxylic acid to achieve good temperature stability and high electrolyte compatibility for use in drilling muds. The polymer blend was made by mixing an alcohol-moist technical grade CMC (carboxymethyl degree of substitution=1.3) made from raw linters and/or wood cellulose with a solution of the sodium salt of a homopolymer of polyacrylic acid and drying the resultant mixture.
Muniov et al. (B. Kh. Muniov, A. A. Alimov, A Abidkhanov and K. S. Akhmedov, Uzb. Khim. Zh., No. 6, 19, 1983) prepared CMC (DS ˜0.8) in a high solids process from raw cotton linters. The CMCs thus made were suggested to be a stabilizer for drilling muds in oil and gas wells.
Although existing grades of CMC and PAC provide a range of performance in drilling fluids, there still is a need for more efficient and economical polymers to function as effective viscosifiers and fluid loss control agents.